1) Field of the Invention
The present invention relates to a technology for automatically controlling a brightness level of a display unit in a portable information device according to the remaining capacity of a battery.
2) Description of the Related Art
In recent years, the portable information devices have been widely used as notebook-size personal computers, personal digital assistants (PDA), electronic personal organizers, or cellular phones, as mobile tools. In particular, the PDA has another function as a terminal dedicated to the Internet, and therefore the PDA is widely used as the mobile tool for managing emails and schedules on individual base. The PDA usually uses a battery in the same manner as that of the notebook-size personal computer as a portable.
Continuous use of the PDA in a state where the remaining capacity of the battery is low causes a problem to occur such that data in use may be lost. Therefore, some measures against the problem are taken by displaying a warning about the remaining capacity of the battery on a display or by sounding an alarm according to the remaining capacity of the battery. In particular, the display unit is provided with a most power-consuming backlight, and as a measure to solve this problem, a technology for controlling the power consumed by the backlight has been disclosed in Japanese Patent Application Laid-Open Publication No. 11-153862. According to the technology disclosed in this publication, the display brightness level of the display unit can freely be controlled through manual operation.
However, the conventional PDA or portable information device has various problems as follows. That is, in the PDA, if the display brightness level of the backlight is increased, then the visibility improves. On the other hand, the increase in the brightness by the backlight results in an increase in power consumption, as explained above.
Conventionally, even if the remaining capacity of the battery is decreased due to continuous use of the PDA, the level (voltage) of the display brightness of the display unit that largely consumes power is not changed and the operation is continued as it is. Therefore, setting of the display brightness level of the display unit to a maximum causes the power consumption of the battery to be increased, thus reducing operating time of the PDA.
Further, considering the power consumed by any device built inside the PDA or provided on periphery of the PDA, a battery voltage drops due to the load of the device when the device is activated (turned on from off). Therefore, a warning about the battery voltage level is frequently displayed because of this abrupt voltage drop, which causes the operating time of the PDA to be extremely reduced as a result.
The display brightness level and transition of the operating time (t) due to operation of the device will be explained below with reference to FIG. 12 and FIG. 13. FIG. 12 shows discharge curves by respective levels of the display brightness in the conventional art. The Y axis shows fluctuations in a battery voltage V, and the X axis shows changes of the operating time (t) by the battery. A battery alarm voltage V1 indicates a level of the voltage with which a warning about a decreased remaining capacity of the battery is given, and a suspend voltage Vs indicates a level of the voltage with which the operation of the PDA is suspended due to the decreased remaining capacity of the battery.
A discharge curve “a” denotes display brightness of the display unit when it is set to a maximum level (MAX), a discharge curve “b” denotes display brightness of the display unit when a normal level (normal), and a discharge curve “c” denotes display brightness of the display unit when a lowest level (dark).
In other words, regarding the operating time of the battery when the display brightness is set to the “maximum” based on the discharge curve “a”, it is found that the remaining capacity of the battery drops to the alarm voltage V1 after t1-second passes and further drops to the suspend voltage Vs after t2-second passes. This t2-second refers to a useful operating time of the battery when the display brightness is set to “MAX”.
Likewise, regarding the operating time of the battery when the display brightness is set to the “normal” and “dark” based on the discharge curves “a” and “c”, it is found that each remaining capacity of the battery in these cases drops to the alarm voltage V1 after t3-second and t5-second pass, respectively, and further drops to the suspend voltage Vs after t4-second and t6-second pass, respectively.
More specifically, when the display brightness is set to the maximum level (MAX), the useful operating time is indicated by t2, when the normal level (normal), the useful operating time is indicated by t5, and when the lowest level (dark), the useful operating time is indicated by t6 (t2<t4<t6). As explained above, because there is a difference between discharge levels of the battery depending on the setting of the display brightness level, a difference is produced between the useful operating times of the portable information device. Particularly, when the display brightness level is set to the maximum, the operating time of the battery-operated portable information device tends to be reduced.
In FIG. 13, reference sign a1 refers to a discharge curve with an abrupt drop when a device is turned on from off. Reference sign a2 refers to a normal discharge curve when a device is not activated. That is, referring to the discharge curve a1 when the display brightness is set to the lowest level (dark) as shown in FIG. 13, it is found that if a device is turned on from off after the time t1 passes, then the battery voltage abruptly drops to a value lower than a battery alarm voltage due to the load.
Therefore, the operating time is disadvantageously reduced to t2 due to a voltage drop in response to turning on of the device though the useful operating time is supposed to be t3 in the normal level. Further, according to the discharge curve a3, it is found that the remaining capacity of the battery instantly drops to the suspend voltage when the load of the device is extremely heavy.